This application claims the priority of German Patent Application Serial No. 101 34 321.3, filed Jul. 14, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates to an apparatus for sealing an end portion of a tubular workpiece in a mold for internal high-pressure forming.
In general, internal high-pressure forming of tubular workpieces involves the use of a suitably contoured mold, which is so configured that a hydraulic high-pressure fluid is normally fed via at least one end portion into the interior space of the workpiece for implementing the forming process. The introduction of the hydraulic fluid may be effected by sealing mandrels which are moved in axial direction by means of hydraulic cylinders into the end portions of the workpiece or in the form of a bell over the end portions, and seal the interior space, in particular with the assistance of gaskets. The hydraulic cylinders together with closed valves oppose hereby the internal pressure applied for implementing the forming process of the workpiece that is carried out after establishing the sealing action.
It is also known to press the sealing mandrels against the end face of the workpiece with the assistance of an annular serrated seal, a stepped seal or a conical seal. It is also conceivable to use the sealing mandrels for forming a conical seal (splined seal).
German Pat. No. 40 17 072 C2 discloses sealing mandrels which are each supported by an additional hydraulic cylinder to produce the same internal pressure that is required for shaping the workpiece so that a counterforce is directly generated. There is only need for the hydraulic unit to apply the differential pressure required for sealing and/or supply of material.
It would be desirable and advantageous to provide an improved apparatus for sealing an end portion of a tubular workpiece in a mold for internal high-pressure forming, which obviates prior art shortcomings and which has a simple construction with less single components, while being capable to significantly reduce clock times for a forming process.
According to one aspect of the present invention, an apparatus for sealing an end portion of a tubular workpiece placed in a mold for internal high-pressure forming of the workpiece, includes a housing having a pressure compartment in fluid communication with an interior space of the workpiece, and a pressure application unit including a pressure source for supply of a hydraulic high-pressure fluid to the pressure compartment, a pressure piston guidingly received in the pressure compartment and defined in the pressure compartment by an effective piston area which is greater than a cross sectional area in the interior space of an end portion of the workpiece, a sealing mandrel at least indirectly connected to the pressure piston and interacting with the end portion of the tubular workpiece, and a check valve disposed in an axial passageway between the pressure compartment and the interior space of the workpiece for blocking in a direction to the pressure compartment.
An essential feature of the present invention is the difference between the effective piston area of the pressure piston in the pressure compartment and the inner cross sectional area in the end portion of the workpiece. In this way, the sealing mandrel applies continuously a force against the end face of the workpiece, as the internal pressure rises in the workpiece, so that application of a required sealing force is ensured, regardless of the respective internal pressure. This sealing force can then be utilized to supply material into the mold or shaping zone of the workpiece.
At the start of the forming process, hydraulic high-pressure fluid from the pressure source is supplied to the pressure compartment. As the passageway in the pressure piston for interconnecting the pressure compartment with the interior space of the workpiece is normally closed off by the check valve in direction of the pressure compartment, pressure builds up in the pressure compartment. As a consequence, the pressure piston is moved toward the end face of the workpiece until the sealing mandrel contacts the workpiece to seal the interior space thereof. As pressure further rises in the pressure compartment, the closing force of the check valve is eventually exceeded so that the check valve opens and pressure fluid is allowed to stream to the interior space of the workpiece. Internal pressure in the workpiece builds up and the workpiece is shaped to conform to the contour of the dies of the mold.
Once the internal high-pressure forming process is over, pressure in the pressure compartment is lowered to zero so that the pressure piston returns to its initial position and the shaped workpiece can be removed for further processing.
Of course, the apparatus according to the present invention is applicable not only one for end of the workpiece but also for both ends.
According to another feature of the present invention, there may be provided a restoring element for moving the pressure piston back to the initial position, when a fluid flow between the pressure source and the pressure compartment is cut. The restoring member may be a mechanical, electrical, pneumatic or hydraulic restoring member. Examples include compression springs, tension springs, pneumatic cylinders or hydraulic cylinders.
Through the configuration of the apparatus according to the invention, there is no need to provide additional hydraulic units or complex control systems to implement the movement of the pressure piston and the sealing action via the sealing mandrel. The movement of the pressure piston and the sealing mandrel can be implemented by the same pressure source that provides the internal pressure for the actual internal high-pressure forming process. Moreover, the sealing action and the application of a counterforce are solely realized from this pressure source. Process times of the internal high-pressure forming operation are significantly reduced because the sealing mandrel is displaced to realize the sealing action as soon as the internal pressure rises.
The apparatus according to the present invention is suitable in particular for hydroforming with mechanical presses for the following reason. During hydroforming operation, pressure buildup is effected in a very short period. Thus, it is sufficient to provide the end face of the sealing mandrel of planar configuration to effect a sealing of the end face of the workpiece and to thereby allow a buildup of the required internal pressure for expanding the workpiece. The planar configuration further allows an expansion directly in the area of the contact surface between the end face of the sealing mandrel and the end face of the workpiece. As a consequence, the workpiece can be directly shaped to the final contour and a subsequent trimming of the ends is avoided.